Hydrogenation process start-up method

ABSTRACT

A START-UP METHOD FOR PROCESS FOR THE LIQUID PHASE SELECTIVE NON-DESTRUCTIVE CATALYTIC HYDROGENATION OF AMETHYLSTYRENE IN A MIXED A-METHYLSTYRENE AND CUMENE FEED WHEREIN UPON THE INITIAL CHARGING OF HYDROGEN AND A-METHYLSTYRENE TO THE HYDROGENATION ZONE CONTAINING A SOLID HYDROGENATION CATALYST THE CONCENTRATION OF HYDROGEN IN THE GASES IN SAID ZONE IS MAINTAINED AT NOT GREATER THAN 40 MOLE PERCENT AND THE INLET TEMPERATURE OF THE MIXED FEEDSTEAM IS MAINTAINED AT NOT GREATER THAN 160* F. UNTIL THE HYDROGENATION REACTION HAS COMMENCED.

United States Patent 3,769,358 HYDROGENATION PROCESS START-UP METHODThomas E. Neta, Houston, and David W. Hill and Robert F. Wiesenborn,Alvin, Tex., assignors to Monsanto Company, St. Louis, M0. N0 Drawing.Filed Mar. 27, 1972, Ser. No. 238,627 Int. Cl. C07c 5/10, 7/00 U.S. Cl.260-667 9 Claims ABSTRACT OF THE DISCLOSURE A start-up method for aprocess for the liquid phase selective non-destructive catalytichydrogenation of amethylstyrene in a mixed ot-methylstyrene and cumenefeed wherein upon the initial charging of hydrogen and a-methylstyreneto the hydrogenation zone containing a solid hydrogenation catalyst theconcentration of hydrogen in the gases in said zone is maintained at notgreater than 40 mole percent and the inlet temperature of the mixedfeedstream is maintained at not greater than 160 F. until thehydrogenation reaction has commenced.

BACKGROUND OF THE INVENTION The present invention relates to ahydrogenation process. More particularly, the present invention relatesto a start-up method in a process for the selective non-destructivehydrogenation of alphamethyl styrene.

The selective hydrogenation of petroleum derived products in anon-destructive manner is well known and widely used throughout the oiland chemical industries. For example, selective hydrogenation is carriedout to remove olefins and diolefins from aromatic feedstocks to preventtheir polymerization and contamination of the products of latertreatment processes and also to facilitate the separation of aromatichydrocarbons from paraffinic hydrocarbons. Such selective hydrogenationprocesses are also employed to convert alkylene aromatic hydrocarbons tothe more saturated alkyl aromatic hydrocarbons. In such a conversion thedesire d product is the alkyl aromatic compound and the hydrogenation iscarried out in a selective manner so as to avoid saturating the aromaticring portion of the hydrocarbon.

The selective hydrogenation of alphamethyl styrene to cumene, orisopropylbenzene, is one such selective hydrogenation process whereinsaturation of the aromatic ring is particularly to be avoided. This istrue, not only because of the loss of desired cumene product occasionedby conversion of the cumene present to isopropylcyclohexane, but alsothe fact that such a saturated compound upon subsequent processing ofthe cumene product, for example by oxidation thereof, producescontaminants which not only lower the yield of the desired oxidationproducts but are very difficult to separate from those oxidationproducts. Consequently, particularly in the process for hydrogenation ofalphamethyl styrene to cumene such hydrogenation of the aromatic ring isto be avoided whenever possible.

It has been found that in the initiation of a process for the selectivehydrogenation of a-methylstyrene feeds, it frequently happens that whensuch feedstock and the hydrogen are first fed to a catalytichydrogenation zone, there occurs a very rapid temperature rise resultingin conditions severe enough to cause hydrogenation of some of thearomatic rings present. This is particularly true in processes whichemploy feedstocks containing both amethylstyrene and cumene as is acommon practice in carrying out this process. This hydrogenation of thearomatic ring is an undesirable result inasmuch as it not only reducesthe amount of cumene product recoverable, but also produces saturatedcyclic compounds which con- 3,769,358 Patented Oct. 30, 1973 taminatethe cumene product and products of its further reaction and treatment.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide an improved process for the selective hydrogenationof a-methylstyrcne in an a-methylstyrene and cumene feed.

Another object of the present invention is to provide a start-up methodfor a process for the selective nondestructive hydrogenation of anu-methylstyrene and cumene feed.

Still another object of the persent invention is to provide a start-upmethod for use in a process for the selective hydrogenation of ana-methylstyrene and cumene feed which prevents rapid temperature risesand hydrogenation of the aromatic rings contained in the feed.

These and other objects of the present invention will become apparentfrom the description below and the appended claims.

Basically, the process of the present invention provides a methodwhereby, in a selective hydrogenation of amethylstyrene in anu-methylstyrene and cumene feed, hydrogenation of aromatic rings in thefeed and temperature runaways are avoided on start-up of the process.According to the method, upon the initial introduction of hydrogen andthe a-methylstyrene and cumene feed into a hydrogenation zone containinga solid hydrogenation catalyst the concentration of hydrogen in thegases present in said zone is maintained in a minor proportion of notgreater than 40 mole percent and the temperature within thehydrogenation zone is maintained low by maintaining the inlettemperature of the feedstream at not greater than F. until thehydrogenation reaction has commenced. After the onset of thehydrogenation reaction the concentration of hydrogen present and theinlet temperature of the feedstream are gradually increased along withincreasing the feed rate of the amethylstyrene and cumene feed untilfull rate steady state hydrogenation conditions are reached.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As explained above, thestart-up method of the present invention is an improvement in theselective hydrogenation process used in the conversion to cumene of theotmethylstyrene present in an a-methylstyrene and cumene feedstream. Theimprovement provided by the present invention and the process ofselective hydrogenation of a-methylstyrene will be described in generalterms below. In the usual hydrogenation process, a-methylstyrene isgenerally present with cumene either by the nature of the source of theot-methylstyrene stream or by mixing such stream with recycled productcumene for the purpose of better temperature control in thehydrogenation reaction. Normally the a-methylstyrene and cumenefeedstream will be comprised of from about 10 to about 60 percentot-methylstyrene, and preefrably from about 15 to about 35 percentthereof. If mixed with recycled product onmene the a-methylstyrenestream prior to mixing with the recycled cumene product normally has acomposition within the following ranges:

Percent by wt. a-Methylstyrene 63-83 Cumene 12-33 Other hydrocarbon andoxygenated derivatives 2-30 into a hydrogenation reactor which containsa solid hydrogenation catalyst. This reactor may be operated in eitheran upflow or downflow manner, but it is generally preferred to operatethe hydrogenation reactor as an upflow reactor. Within the hydrogenationreactor temperatue and pressure conditions are maintained such as tokeep the a-methylstyrene and cumene feed in the liquid phase during itspassage therethrough. Normal temperatures within the reactor willgenerally range from about 100 to 500 F. and preferably from 140 to 380F., whereas pressures will be maintained within the range of 100 to 800p.s.i.g. and more preferably from 150 to 400 p.s.i.g. The hydrogenationcatalyst contained in the reactor will be one of the well-known solidhydrogenation.

catalysts such as a noble metal deposited on an alumina support.

Concurrently with the introduction of the mixed stream ofu-methylstyrene and cumene, hydrogen is also introduced into thehydrogenation reactor. During normal operation of such process it is notnecessary that the hydrogen stream introduced be pure hydrogen, but itis preferred for reasons of economy that it be a stream containing atleast 75 mole percent hydrogen. The amount of hydrogen introduced intothe reactor during normal operation varies somewhat depending on thehourly space velocity of the a-methylstyrene and cumene feed beingintroduced and the composition of such feed.

In the hydrogenation reactor the a-methylstyrene present in the mixeda-methylstyrene and cumene feed is selectively hydrogenated to cumeneand a cumene product is removed from the reactor. Such product isremoved overhead from the reactor in the case of upfiow operationthereof. Some of the cumene product stream is recycled to the feed lineto the hydrogenation reactor to supply recycled cumene thereto, and theremaining portion representing the cumene product is purified, stored,sold and/ or forwarded to further treatment and reactions such asoxidation.

In the general process just described it has been found that uponstart-up of the process, such as when the catalyst has been regeneratedor replaced and the process is put back into operation or when theprocess is started up for the first time in a new installation, sharptemperature rises or temperature runaways" are encountered in thereaction with the result that significant quantities of saturated cycliccompounds, chiefly isopropylcyclohexane, are produced by hydrogenationof the aromatic rings in the cumene either present in the feed stream orthat produced by selective hydrogenation. As explained above not only isthe yield of the desired cumene product thus reduced, but theundesirable saturated cyclic by-products cause difiiculties in laterprocessing of the cument product.

In accordance with the start-up method of the present invention, thehydrogenation reactor is first pressured with an inert gas prior tointroduction of either the a-methylstyrene and cumene feed or hydrogen.This initial pressuring of the hydrogenation reactor serves twopurposes. The pressuring with an inert gas serves as a safety measure toexclude air or oxygen from the reactor. Of much greater importance, thepresence in the reactor of the inert gas serves to insure that theconcentration is hydrogen in the gaseous phase in such reactor ismaintained at a value of not greater than 40 mole percent until afterthe initiation of the hydrogenation reaction.

Pressures of an inert gas of between and 200 p.s.i.g. in the reactor aregenerally sufficient both to insure that any air or oxygen has beendisplaced as well as to insure the necessary degree of dilution of thehydrogen initially introduced. Generally, a pressure of about 90 to 100p.s.i.g. is preferred. The term inert gas as used herein refers to anygas which is unreactive under the conditions of the process, and morespecifically refers to one of the gases commonly used for blanketing,purging, etc. Gases included in this group are the light hydrocarbongases such as methane and ethane, nitrogen, argon, helium and the like.Practically any gas which does not react with the catalyst, thea-methylstyrene and cumene feed or the hydrogen can be used topressurize the reactor in the present start-up method. Usually, becauseof its availability and economy, nitrogen is the inert gas chosen.

After the hydrogenation reactor is pressured with the inert gas, thereis established a flow of recycled cumene product which is heated toapproximately F. When the reactor has reached the temperature of thecumene feed, usually about 150 F. the inert gas pressure is raised tothe normal operating level of about 165 to 220 p.s.i.g. Then asufiicient amount of hydrogen is charged to establish a hydrogenconcentration in gaseous phase of approximately 30 to 35 mole percent,but such concentration is limited to no greater than 40 mole percent.

Thereafter the introduction of the a-methylstyrene is commenced byintroduction thereof into the feed stream line together with therecycled cumene. During the start of the hydrogenation reaction, thetemperature of the feed stream of a-methylstyrene and cumene ismaintained at no greater than F. and preferably at about 150 F.

Upon introduction of the a-methylstyrene stream and the resultant onsetof hydrogenation, the concentration in the gaseous phase is maintainedat no greater than 40 mole percent by introducing hydrogen into therecycled inert gas in sufiicient quantity to maintain suchconcentration. The gas entering the reactor is sampled and analyzed todetermine the amount of make-up hydrogen required. The hydrogen added tothe recycled inert gas is generally a stream containing at least 75 molepercent and preferably about 90 mole percent of hydrogen.

The liquid hourly space velocity of the u-methylstyrene feed stream isinitially held at a relatively low value of about from .027 to .134 perhour and preferably about .080 to .107 per hour while the recycledcumene flow rate is maintained at from a 5:1 to 6:1 ratio by volume tothe a-rnethylstyrene feed stream. When employing the startup method ofthe present invention, no temperature kick or exotherm is experiencedand the onset of hydrogenation is accomplished under the relatively mildconditions initially established.

When the hydrogenation has been established and analysis of the productcumene demonstrates hydrogenation of essentially all the a-methylstyreneto cumene in the initial charge, the flows of u-methylstyrene, hydrogenand cumene are gradually increased up to full steady state rates. Duringsuch gradual increase, the temperature of the feed stream at the reactorinlet and the concentration of hydrogen in the gaseous phase arelikewise gradually increased while the pressure in the reactor ismaintained at approximately the level at which the hydrogenation wasinitiated, about to 220 p.s.i.g. The rate of a-methylstyrene addition isincreased at a LHSV rate of .080 to .107 per hour until it representsfrom 15 to 30 percent and preferably about 20 percent of the feedstream. Thereafter the flow rates of both the a-methylstyrene and cumeneare increased up to full steady state rates so as to maintain thispercentage composition of the feed stream. At the same time, thetemperature of the feed stream at the reactor inlet is increased at arate of not greater than 5 F. per hour and preferably 2 to 3 F. per houruntil operating temperatures of about to 205 F. but in no event inexcess of 225 F. are established.

Likewise, during the period of gradual increase in rates, theconcentration of hydrogen in the gaseous phase is graduaully increasedat a rate of about 5 to 10 mole percent per hour and preferably at about5 mole percent per hour until the inert gas is finally purged from thesystem and the hydrogen feed is charged at full purity of from 75 to 90or more percent hydrogen as desired. At periods during the increase tofull rate operation, the product cumene is sampled and analyzed toinsure that essentially all the a-methylstyrene is hydrogenated tocumene. If the concentration of unreacted a-methylstyrene in the productstream is found to rise above the initial trace quantities,

then the hydrogen concentration and/or the feed stream inlet temperaturecan be raised by the next increment to secure essentially completehydrogenation of such amethylstyrene.

When carrying out the start-up procedure in the manner described above,analysis of the product cumene stream has established that only verysmall or virtually trace quantities of isopropylcyclohexane or othercyclic saturated compounds are produced compared to starting up withhydrogen of higher concentration or with a feed stream at higher reactorinlet temperatures.

As noted above, the catalyst contained in the reactor can be any of thewell known solid hydrogenation catalysts. Although not required, it isgenerally preferred that porous, particle form supports be employed forthe catalyst to adquately disperse and increase the surface area of theactual catalytic agent. Such porous supports may comprise natural ortreated clays, such as kaolin or bentonite; clay-like materials, such asCelite or Sil-O-Cel;- and synthetic materials, such as magnesium oxide,silica gel, alumina gel, zeolites, activated carbon or diatomaceousearth and the like. Activated alumina, which is the well knowncrystalline alpha alumina monohydrate, or chi alumina are quitesatisfactory and preferred as porous catalyst supports. The catalyticagent employed can be any of the noble metals in Group VIII ofthePeriodic Table with atomic numbers of at least 27, such as rhodium,palladium and platinum; and also cobalt, nickel, either supported orunsupported, and the like. Because of their high hydrogenation activityat low temperatures, the preferred catalylst are palladium or platinumon alumina supports, with palladium on activated alumina support themost desirable catalyst. When the catalyst comprises one of the noblemetals on a support, the noble metal will generally be present withinthe range of 0.1 to 5% and preferably 0.2 to 1% by weight of thecatalyst.

The following examples will serve to illustrate the present inventionand demonstrate the utility of the present start-up method.

The examples were carried outwith an rat-methylstyrene stream of thefollowing approximate composition which, in normal operation, is mixedin a ratio of 1:3 with a stream of recycled cumene.

Percent by wt.

Example I Using a system such as that described above, a reactorcontaining a solid hydrogenation catalyst comprised of an aluminasupport and containing 0.3 weight percent of palladium was utilized forhydrogenation of a-methylstyrene employing the start-up procedure asdescribed. The reactor was pressured with nitrogen to approximately 100p.s.i.g. At this point, a stream of recycled cumene product at 150 F.was admitted to the reactor. Following this additional nitrogen wasadmitted to the reactor in an amount to increase the reactor pressure to175 p.s.i.g. Hydrogen was then admitted to establish a hydrogenconcentration of approximately 26 mole percent as determined by analysisof the gas. Thereupon, introduction of a stream of rat-methylstyrene wasbegun and hydrogenation initiated. No temperature rises were noted uponcommencing hydrogenation or the subsequent gradual increases in feedrates of (rt-methylstyrene, cumene and hydrogen and increases in thefeed stream inlet temperature and hydrogen concentration. The feedrates, temperatures and concentration of hydrogen as the reaction wasbrought to steady state conditions are set out in Table 1 below.Analysis of the cumene product stream for unreacted umethylstyrene andisopropylcyclohexane as well as of the TAB LE 1 Cumenel AMS streams, STeiprg, Pressure,

AM Percent by vol. LHSV/hr. p.s.i.g. H2

OOOOOOOOOOOOHOOOOOOOUIOO In contrast to the above results, it was foundthat upon start-up of the same reactor system containing fresh catalystfor hydrogenation of u-methylstyrene at gradually increased feed rates,but with the inlet temperature of the (at-methylstyrene and cumene feedinitially at 225 F. and the concentration of hydrogen in the reactor gasphase at 70 to mole percent there was produced a cumene product streamcontaining approximately 12 percent by weight of isopropylcyclohexaneover the first 24 hours of such start-up. Likewise, when a similarstart-up of fresh catalyst in the same reactor was carried out with theinitial inlet temperature of the rat-methylstyrene and cumene feed atabout 195 F. and the hydrogen concentration at approximately 70 molepercent there was produced approximately 6 percent by weight ofisopropylcyclohexane in the cumene product stream over the first 24hours of operation.

The necessity for maintaining the concentration of hydrogen at a valuenot greater than 40 mole percent as well as maintaining a relatively lowinitial inlet temperature of the feed stream in order to realize theadvantages of present start-up procedure is demonstrated by thefollowing example.

Example II The same general start-up procedure as described above inExample I employing a fresh charge of the same catalyst as used thereinwas carried out. However, in this instance the initial temperature ofthe u-methylstyrene and cumene feed was at about 185 F., although it wasrapidly decreased to about F. In addition, the hydrogen concentration atthe initiation of hydrogenation was greater than 80 mole percent. Thiswas indicated by the fact that after only 18 hours of operation theanalysis TABLE 3 Percent AMS What is claimed is:

1. In a process for the selective non-destructive hydrogenation ofa-methylstyrene in an a-methylstyrene and cumene feed wherein saida-methylstyrene and cumene feed in the liquid phase and hydrogen areintroduced into a hydrogenation zone containing a solid hydrogenationcatalyst, selected from the group comprising a noble metal of Group VIIIof the Periodic Table having an atomic number of at least 27, cobalt ornickel, the improvement whereby saturation of aromatic rings in saidfeed and excessive temperatures are prevented in the initiation of saidprocess, said improvement comprising (a) pressuring said hydrogenationzone with an inert gas unreactive under the conditions of the process,prior to introduction of hydrogen thereto,

(b) maintaining the concentration of hydrogen to inert gas in saidhydrogenation zone at not greater than 40 mole percent until after theinitiation of the hydrogenation of a-methylstyrene, and

(c) maintaining the inlet temperature of said feed stream at not greaterthan 160 F. until after the initiation of the hydrogenation ofa-methylstyrene.

2. The process of claim 1 wherein said catalyst comprises from 0.1 to 5percent by weight of palladium or platinum supported on alumina.

3. The process of claim 1 wherein said a-methylstyrene and cumene feedcomprises from 10 to percent by Weight m-methylstyrene.

4. The process of claim 1 wherein said inert gas is nitrogen.

5. The process of claim 1 wherein said u-methylstyrene and cumene feedand said hydrogen are maintained in contact with said catalyst at atemperature of from about to about 500 F. and at a pressure of fromabout 100 to 800 p.s.i.g.

6. The process of claim 1 wherein said concentration of hydrogen ininert gas in maintained at about 30 mole percent until after initiationof hydrogenation.

7. The process of claim 1 wherein said inlet temperature is maintainedat about 150 F. until after initiation of hydrogenation.

8. The process of claim 1 wherein said a-methylstyrene and cumene feedand said hydrogen are maintained in contact with said catalyst at atemperature of from to 380 F. and at a pressure of from to 400 p.s.1.g.

9. The process of claim 1 wherein after the initiation of hydrogenationof a-methylstyrene the concentration of hydrogen in said hydrogenationzone is increased at a rate of not greater than 5 mole percent per houruntil such concentration is at least 75 mole percent and the said inlettemperature of the said a-methylstyrene and cumene feed is increased ata rate of not greater than 5 F. per hour to an inlet temperature of atleast F.

References Cited UNITED STATES PATENTS 3,127,452 3/ 1964 Codignola260-667 3,433,845 3/1969 Kovach et al 260--667 DELBERT E. GANTZ, PrimaryExaminer J. M. NELSON, Assistant Examiner US. Cl. X.R.

